"The Heartbreak Kid" is a Farrelly Bros. comedy mixed with that of someone holding a grudge. Though the directors are known for specializing in raunchy humor in films such as 1998's "There's Something About Mary," 2000's "Me, Myself & Irene" and 2001's "Shallow Hal," there has always been an unabashed sweetness and goodness of heart underneath the salacious slapstick. Not this time. True to form, "The Heartbreak Kid" starts off well, with a first hour that is raucous and lively and savvily scripted, but then a sharp turn for the worst is made. Suddenly, the near-consistent laughs are replaced with asinine, insanely idiotic plotting and a mean-spirited, borderline-irresponsible undercurrent that botches the entire experience. By the end, the viewer not only disagrees with the actions of the person who is meant to be rooted for, but actively hates him and feels sorry for all the victims he has hurt along the way.

Eddie Cantrow (Ben Stiller) is a never-married San Francisco-based singleton who, at forty years old, has begun to wonder if he will ever find his true love. He finally thinks he has found her in the breathtaking form of Lila (Malin Akerman), an environmental researcher he helps out after her purse is snatched. Their courtship is a whirlwind one, and in six weeks they have married. From the moment their fairy-tale honeymoon to Cabo San Lucas, Mexico, begins, Lila's unattractive and offbeat characteristics rear their ugly heads, from a bout of queefing, to a desire for aggressive sex, to a deviated septum in her nose, to a tendency to sing every song that comes on the radio.

With Lila eventually holed up in their hotel room after getting a nasty sunburn, Eddie begins to question whether or not he has made a mistake. Helping him out is the free-spirited Miranda (Michelle Monaghan), a Mississippi school teacher vacationing with her family at the same resort. She has no idea Eddie is marriedevery time he tries to tell her, something gets in the way, natchbut the more time they spend together, the more he is convinced she is the right woman for him. The audience, meanwhile, is not so sure.

Loosely based on a 1972 picture starring Charles Grodin, "The Heartbreak Kid" has been written and directed by Bobby and Peter Farrelly and co-written by a staggering three more committee membersScot Armstrong (2006's "School for Scoundrels"), Leslie Dixon (2007's "Hairspray") and first-timer Kevin Barnett. Among all of them, it is difficult to believe that no one spoke up about the destructively dumb story turns, gaping plot holes and sheer unpleasantness of the lead protagonist. Before these unfortunate elements get underway, the film is pretty much a delighta goofy and entertaining adult-oriented romance about a man who gets more than he bargained for with the virtual stranger he has been betrothed to. By taking the time to invest in the relationship between Eddie and Lila, you find yourself caring about them, which makes what comes next so hilarious.

From Lila's over-the-top car sing-alongs to her rough gymnastics in the sack to her bad math skills (seeing an elderly couple in a restaurant, she gushes to Eddie how that is going to be them in ten years), it is plain to see how Eddie is a bit taken aback by this woman he only thought he knew. Nevertheless, Lila is not a hateful character. Sure, she has her individual peculiarities and flaws, but that is what makes her blessedly human and, dare it be said, even more likable. For Eddie to so abruptly throw Lila away like yesterday's news is indefensible, and for him to string along the equally lovely Miranda without filling her in on his marriage is just as unsavory.

By this time, the narrative becomes increasingly strained and ludicrous and the comedy softens to a dull hum. The third acttake a deep breathinvolves a silly "Three's Company"-style misunderstanding, followed by a hideously unnecessary and illogical sequence involving Mexicans sneaking into the U.S., followed by a misbegotten bedside declaration that could have been handled with a simple e-mail, followed by a faux-upbeat ending that is cruelly distasteful, followed by a mid-end credits scene that spits on a character who doesn't deserve it while making bizarre light of bestiality. The five writers couldn't have come up with worse ideas if they tried. More specifically, what Bobby and Peter Farrelly were thinking is anyone's guess.

The performances are too astute for the imbecilic material they are given. Ben Stiller (2006's "A Night at the Museum") is amiable as Eddie up until a point when his characterthe hero of the piece, mind youturns into a bully with no regard for anyone's feelings but his own. He deserves neither Lila nor Miranda, and the only happy conclusion would have been one that saw Eddie sad and alone. Michelle Monaghan (2006's "Mission: Impossible III") is an easy-breezy charmer as Miranda, her slight southern accent a low-key but valuable tool. As Lila, Malin Akerman (2007's "The Brothers Solomon") steals the show. Her sharpened comic skills remind one of Cameron Diaz, but with her own specialized originality and flair. Akerman is fearless in the things she is asked to do, and among it all manages to still be a sympathetic presence. Then again, perhaps if Lila was more of a monster, the audience would be able to swallow how Eddie treats her. As is, she is a character to feel sorry for. In supporting parts, Jerry Stiller (2002's "Serving Sara") chews up his foul-mouthed dialogue as Eddie's father and Rob Corddry (2007's "Blades of Glory") gets some memorable moments as Eddie's hen-pecked best friend Mac.

"The Heartbreak Kid" is a huge disappointment, especially coming from a usually reliable sibling team such as the Farrellys. In aiming for the wild and gross-out gags of their earlier movies, they have misplaced any semblance of humanity in the way they unspool the story and treat their characters. Locale lensing in San Francisco and Mexico by cinematographer Matthew F. Leonetti (2007's "Pride") is a warm and inviting contrast to the stupidity and malice that follows. "The Heartbreak Kid" is not the romantic comedy it is selling itself as, but a horror story about a selfish guy getting exactly what he wants and not earning any of it. There is no fun in that, only misery.